I could've sworn I saw a post somewhere on these forums recently that had a link to a "normal app or script" version of the ePubCheck Google Code thing. Was I hallucinating? Or is the only way to run ePubCheck on a Mac via the command line? (or from within Sigil or Oxygen)?

There is somewhere on this forum a link for Apple, I remember that.
You cannot run ePubCheck from within Sigil. Sigil has its own validation tool which is a lot better than ePubCheck. The validation from Sigil, called FlightCrew, can also be installed standalone.